1. Field of the Invention
The present invention relates to a protective coating for an article exposed to hot, aggressive gas flows and, more particularly, to a layer for bonding a protective layer to a gas turbine engine component.
2. Description of Related Art
Gases flowing through a turbine engine reach extremely high temperatures and velocities. It is a significant engineering challenge to build components that will withstand the impingement of a high velocity gas at temperatures that can exceed 1000xc2x0 C. The demands on an engine""s turbine blades are particularly extreme, because they are exposed to high velocity, high temperature gases while being subjected to the forces resulting from rotation at thousands of revolutions per minute.
Prior art turbine blades are typically a laminated structure, with a so-called superalloy substrate having a heat resistant coating. These superalloys are typically cobalt- or nickel-based materials, and the protective coatings have taken a variety of forms. One known component of such coatings is an MCrAlY alloy, where Cr is chromium, Al is aluminum and Y is yttrium and/or a rare-earth element, with the remainder M selected from the group consisting of iron, cobalt, nickel or mixtures thereof.
EP 486 489 (U.S. Pat. No. 5,401,307) is an example of a prior art corrosion-resistant protective coating for medium and high temperatures up to about 1050xc2x0 C. for a gas turbine part made of a nickel-based or cobalt-based alloy. The protective coating has, in percentages by weight, 25% to 40% nickel, 28% to 32% chromium, 7% to 9% aluminum, 1% to 2% silicon and 0.3% to 1% of at least one reactive rare-earth element, at least 5% cobalt and selectively from 0 to 15% of at least one element from the group consisting of rhenium, platinum, palladium, zirconium, manganese, tungsten, titanium, molybdenum, niobium, iron, hafnium and tantalum. In the disclosed embodiments, the protective coating merely comprises the elements nickel, chromium, aluminum, silicon, yttrium and additional rhenium in a range of 1% to 15%, the remainder being made up of cobalt. The corrosion properties are improved considerably by the addition of rhenium.
U.S. Pat. No. 4,321,310 is another example of such prior art. It describes a gas turbine component which has a base body made of the nickel-based superalloy MAR-M-200. A layer of an MCrAlY alloy, in particular an NiCoCrAlY alloy, having 18% chromium, 23% cobalt, 12.5% aluminum and 0.3% yttrium, with the remainder being made up of nickel, is applied to the base material. This alloy layer has a polished surface, to which an aluminum oxide layer is applied. A ceramic thermal insulation layer, which has a columnar structure, is applied to this aluminum oxide layer.
U.S. Pat. No. 4,585,481 likewise uses protective layers for protecting a superalloy metallic substrate against high-temperature oxidation and corrosion. MCrAlY alloys are employed for the protective layers, and the patent discloses such layers with 5% to 40% chromium, 8% to 35% aluminum, 0.1% to 2% of an oxygen-active element from group IIIb of the periodic table, including the lanthanides and actinides and mixtures thereof, 0.1% to 7% silicon and 0.1% to 3% hafnium, the remainder being made up of nickel and/or cobalt. The corresponding protective layers made of MCrAlY alloys are, according to this patent, applied using a plasma-spray method.
Yagodkin, Y. D., et al., xe2x80x9cApplication of Ion-Beam Treatment in Turbine Blade Production Technology,xe2x80x9d Surface and Coatings Technology, Vol. 84, pp. 590-592 (1996), disclose doping protective layers of NiCrAlY- or NiCoCrAlY-type alloys with boron or lanthanum by exposure to ion beams. The protective layers are in this case applied to a nickel-based superalloy. No information has been found in this article regarding the magnitude of the possible degree of doping.
WO 96/35826 Al discloses a thermal insulation layer for a superalloy turbine rotor blade that is exposed to hot gas during operation. The thermal insulation layer has a ceramic protective layer bonded to an adhesion layer made up of an aluminide or an MCrAlY-type alloy. The superalloy may either be MAR-M247, a nickel-based alloy with a proportion of hafnium, or MAR-M509, a cobalt-based alloy with a proportion of zirconium. The adhesion layer is an MCrAlY alloy having a content of 10% to 35% chromium, 5% to 15% aluminum and 0.01% to 1% of one of the elements yttrium, hafnium or lanthanum, that lies over the superalloy substrate. According to this document, the oxidation of aluminum in the adhesion layer gives rise to an aluminum oxide layer which makes it possible for the ceramic protective layer to bind to the adhesion layer. In this case, partially stabilized zirconium is used as the ceramic protective layer, such stabilization being carried out using calcium oxide, magnesium oxide, cerium oxide or yttrium oxide.
However, those skilled in the art still seek a better system to protect articles subjected to erosion by hot, aggressive gases, such as gas turbine engine components.
It is an object of the present invention to avoid the shortcomings of prior art structure for protecting articles in demanding environments, and particularly to provide a layer system for protecting gas turbine engine components such as turbine blades.
It is another object of the present invention to provide a product having a metallic base body having thereon a bonding layer with an alloy for binding to it a thermal insulation layer, particularly one comprising a ternary oxide.
In furtherance of the objects of the present invention, one aspect of the invention involves an article having a metallic substrate with a layer system thereon including a ceramic thermal insulation layer and a bonding layer between the ceramic thermal insulation layer and the substrate, wherein the bonding layer is an alloy comprising at least one element from the group comprising iron, cobalt and nickel, and the elements of at least one of a group 1 and a group 2, wherein:
group 1 comprises the following elements (expressed in percentages by weight):
Chromium: 3% to 50%
Aluminum: 3% to 20%
Yttrium and/or a rare-earth element: 0.01% to 0.5%
Lanthanum: 0.1% to 10%
Hafnium: 0 to 10%
Magnesium: 0 to 10%
Silicon: 0 to 2%; and
group 2 comprises the following elements (expressed in percentages by weight):
Chromium: 3% to 50%
Aluminum: 3% to 20%
Yttrium and/or a rare-earth element: 0 to 0.5%
Lanthanum: 0.1% to 10%
Hafnium: 0.1% to 10%
Magnesium: 0 to 10%
Silicon: 0 to 2%.
The invention is particularly adapted for use with a component of a gas turbine engine such as a turbine blade, a guide vane or a heat shield element, in which the component is a nickel- or cobalt-based alloy.